[Therion] scaling a line thickness in Metapost

[Tarquin Wilton-Jones]

I want to be able to select a different thickness of line for different
"scale" values that can be selected in the .th2 file. Lines can have a
scale like this, which seems perfect for what I need:
-scale xs

* Is there a way to access this value from within a custom line
definition like this?
def l_u_foo (expr P)=
if ATTR_scale ...

* Assuming it can be seen from some ATTR value, internal variabe or
function parameter, what values will it see for the various -scale
options (xs, s, m, l, xl) - if it is a number, what are the numeric
values for the various scales, or if it is a string, does it just see
the literal values "xs", "s" etc.?

Cheers!

Tarquin